In general, radial tires widely in use are provided with a stiff belt disposed radially outside the carcass. On the other hand, tire performances, for example steering stability, durability, ride comfort and the like are greatly altered by the characteristics of such-belt, for example, belt rigidity. Therefore, it is a very important point in designing tires to adjust such belt characteristics.
Heretofore, such a belt has been made of a steel cord ply or an organic fiber cord ply or a combination thereof as shown in FIGS. 6-8.
FIG. 6 shows a belt layer (A) composed of steel cord plies (a1). In this case, the belt weight becomes heavy, which deteriorates dynamic performance and fuel consumption performance. Further, the tread rigidity is liable to become excessively high in the tread crown region, which deteriorates ride comfort and disturbs the ground pressure distribution.
FIG. 7 shows a belt layer (A) composed of aromatic polyamide cord plies (a2). In this case, the belt weight can be reduced in comparison with the steel cord belt, and the ride comfort is superior to the steel belt tire. However, steering stability is inferior in relation thereto. Further, as the aromatic polyamide cords are poor in buckling strength, the cords are liable to be broken, and separation failure from the surrounding rubber is also liable to occur with decreasing tire durability.
FIG.8 shows a belt layer (A) composed of a steel cord ply (a1) and an aromatic polyamide cord ply (a2) disposed on the radially outside of the steel cord ply. In this case, there is a large difference in rigidity between the different plies and further the aromatic polyamide cord ply (a2) is poor in adhesion to rubber. Therefore, ply separation failure is liable to occur.
Thus, it is very difficult to improve tire performance all around by using such a conventional belt ply made of single material cords, since the characteristics of the ply are derived from the nature of the cords used, and the characteristics are greatly altered if the cord material is differed. Further, design freedom is restricted.